LIGA (Lithographie Galvanik Abformung) technology, developed in the 1980s by W. Ehrfeld of the Karlsruhe Nuclear Research Centre, Germany, has proved advantageous for the fabrication of high precision metallic microstructures.
The principle of the LIGA technique consists in depositing, on a conductive substrate or substrate coated with a conductive layer, a layer of photosensitive resin; in performing X-irradiation through a mask matching the contour of the desired microstructure, by means of a synchrotron; in developing, i.e. removing by physical or chemical means, the non-irradiated portions of the photosensitive resin layer, so as to define a mould having the contour of the microstructure; in the galvanic deposition of a metal, typically nickel, in the photosensitive resin mould and then removing the mould to release the microstructure.
The quality of the microstructures obtained is beyond reproach, but the requirement to implement an expensive piece of equipment (the synchrotron) makes this technique incompatible with the mass production of microstructures that have a low unit cost.
This is why similar methods, based on the LIGA method but using UV photosensitive resins, have been developed. A method of this type is described, for example, in the publication by A. B. Frazier et al. entitled “Metallic Microstructures Fabricated Using Photosensitive Polyimide Electroplating Molds”, Journal of Microelectromechanical systems, Vol. 2, N deg. 2, June 1993 for fabricating metallic structures by electroplating metal in photosensitive polyimide based moulds. This method includes the following steps:                creating a sacrificial metallic layer and a seed layer for a subsequent electroplating step;        applying a photosensitive polyimide resin layer;        exposing the polyimide resin layer to UV radiation through a mask matching the contour of the desired microstructure;        developing, by dissolving, the non-irradiated parts of the polyimide layer so as to obtain a plurality of polyimide moulds;        galvanic deposition of nickel in the open parts of the moulds up to the height of said moulds;        separating the metallic structures obtained from the substrate; and        removing the polyimide moulds to release the electroformed metallic parts.        
The electroformed microstructures or parts are thus obtained in bulk. Once obtained, these parts are separated and have to be bonded back to a plate in order to be machined and/or ground to the desired thickness and surface state.
These steps require lengthy handling time and include significant risks of the parts being arranged the wrong way on said plate, in particular when the electroformed parts have small dimensions, typically parts with dimensions of less than a millimeter. These methods involve a scrap rate and thus production costs which are incompatible with the requirements of an industrial method.
Moreover, the methods of the prior art require a deposition of electroformed material that is sufficiently large to ensure that all of the parts attain their minimum thickness regardless of variations in thickness of the resin at the substrate surface. This thus leads to a waste of electroplated material.
Indeed, the thickness variations in the resin deposited to form the moulds are intrinsic to current deposition methods, typically spin or spray coating. It will be noted in this regard that the non-uniformity of the resin layer in which the moulds are formed means that the resin has to be irradiated with a setting that takes account of the maximum and minimum thickness. This leads to an increase in the dispersion of geometric dimensions in the plane of the moulds.
There therefore exists a requirement for a method that overcomes these drawbacks.